The invention relates to a variable-shaft-diameter ridge-imitating cassava stem smashing and returning machine which comprises a machine frame, a three-point suspension mechanism, a transmission device, a smashing and returning device and two land wheels, wherein the machine frame is installed behind a tractor through the three-point suspension mechanism, and an output shaft outputting power behind the tractor provides power for a transmission system. The smashing and returning device comprises a variable-shaft-diameter smashing cutter shaft, a smashing cutter holder, a first bearing seat support, a second bearing seat support and a profiling smashing cavity; and the variable-shaft-diameter smashing cutter shaft comprises a middle small-shaft-diameter cutter shaft and large-shaft-diameter cutter shafts at the two ends. A variable-shaft-diameter ridge-imitating structure is adopted, the problems that many cassava stems remain in furrows and smashing is not thorough are solved, the service life of a cutter set is effectively prolonged, the hitting force of the furrows is increased, the whole machine structure is simplified, energy consumption is reduced, the operation stability and safety are improved, waste cassava stems are effectively prevented from sprouting again, the cassava harvesting cost and harvesting efficiency are reduced, and smashing capacity and working efficiency are improved.